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Thread: Vacuum Forming Splitter

  1. #1
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    Default Vacuum Forming Splitter

    After an encounter with a dirt berm, I need to make a new splitter. I'm considering using vacuum formed ABS sheet over a plywood mold for the splitter, then a second piece for the air dam bonded to the top. After my experience with the dirt berm I'd rather have the splitter break and not take anything else with it. Anyone else vacuum formed something as large as a splitter?

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    Tegris dude... See if you can still find some NASCAR COT splitters. I picked up a bunch of them a few years ago for like $10 each and made my splitter out of one of them. yeah it weighs 10lbs, but dirt? Don't care!
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    The reason I was thinking ABS is because that's what it's mounted to, and won't take out the bumper cover/undertray next time it hits something. Also, ABS vacuum forming looks like a good way to easily make a new one. I'm thinking I might make another ply wood splitter then make a mold and replicate in G-FRP. But ABS vacuum forming appears to be simple and quick.

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    how is your splitter mounted? we use a "L" shaped bracket bolted to transport tie-down locations on the bumper horns, the bottom of which has a slot at the front, opening to the rear, and a pin hole at the back. the splitter plate has mounts which capture the brackets in double shear, so the slotted mount "hole" never has to come undone. use a small bolt for the fixed mount and a 3/8" or so up front. one mount on each side. if you hit something, the rear pin shears and the splitter falls off, most if not completely intact. make it from good plywood covered in a thin layer of GFPR for toughness.

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    Quote Originally Posted by Chip42 View Post
    how is your splitter mounted? we use a "L" shaped bracket bolted to transport tie-down locations on the bumper horns, the bottom of which has a slot at the front, opening to the rear, and a pin hole at the back. the splitter plate has mounts which capture the brackets in double shear, so the slotted mount "hole" never has to come undone. use a small bolt for the fixed mount and a 3/8" or so up front. one mount on each side. if you hit something, the rear pin shears and the splitter falls off, most if not completely intact. make it from good plywood covered in a thin layer of GFPR for toughness.
    Do you have any pics you'd be willing to share? My methods sound somewhat similar, but I've not yet figured out how accomplish the graceful departure.

    What's GFPR?
    Marty Doane
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    Quote Originally Posted by Eagle7 View Post
    Do you have any pics you'd be willing to share? My methods sound somewhat similar, but I've not yet figured out how accomplish the graceful departure.

    What's GFPR?
    shorthand for fiberglass(glass fiber reinforced plastic)

    I have some CAD I could screenshot and post, on car is not possible right now. hard part is a pin with enough strength to hold the splitter and "desired" loads while failing in a collision. also, if you add stays, they have to be able to give way OR allow the movement of the splitter plank.


    the shape is structurally compromised to allow MR2 coolant hose routing, but you can see the top bolt locations which allow this to replace the OEM tie-down hooks, then the forward slot and rear pin-hole - that's 3/8" now and probably doesn't fail as intended (well, not without ancillary damages) but as is it certainly makes installation and removal easier. high hole on the front is to add a stay if needed, like a steel cable X across the radiator to the bumper mounts. the bottom part is pretty universal once you have a place to mount it. I'm happy to help draw or make one if you like.
    Last edited by Chip42; 03-06-2014 at 02:21 PM.

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    I had a combination of carriage head bolts and screws to the ABS bumper cover and the aluminum under tray. It wound up breaking into three parts. The center peeled the aluminum undertray into a ball under the car and moving the oil cooler line into the alternator pully. The two corner stayed some what bolted on, except the drivers side finder liner was part of the aluminum ball, along with my unharmed wired transponder. Since I have to install a new bumper cover, I could bolt some mounts with break-away points onto the aluminum bumper bar, but was thinking of swiss cheesing the bar for nose weight reduction. What I want is something that's easy to replace, and doesn't take anything else (bumper cover, under-tray, or finder liners) out when it goes away.

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    plywood, attached from the top up to something like I showed will allow the splitter plank to break/break off without taking the bumper and other bits with it.

    treat the splitter plate as disposable / expendable and you are halfway there.

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    Conceptually, that's exactly what I've got now, except I've got the front slot facing forward - which of course eliminates the possibility of a clean break. It took a big hit last summer and didn't fare well.

    Do you have ideas for a rear pin that will shear when you want but not when you don't? Maybe something non-metalic and brittle? Or plastic?
    Marty Doane
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    Whatever you choose it has to be strong enough to support the aero load and light dings but break free at a somewhat limitted contact force.

    Pins dont really cover those criteria if you are making good aero use of the thing, ive been thinking about T6 aluminum flat stock, notched. But i have to run the math to see how much to notch and what cross section does best. Its not as easy to use as a bolt, but should give the desired results.

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    What about shear bolts from a snowthrower? Made to shear under sharp force and you can buy them in six-packs at Lowe's. Just gotta design it so the shear plane is in the right location. - GA
    Not my circus...not my monkeys...

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    my thinking is that you need to have the aero-loading capability and the breakaway, so a pin, being equal strength in both rearward and downward loads (loaded in shear, mounted laterally) would need to be stronger than you might want for impact in order to be strong enough for use in general. but then there's the notion of orienting the bolt vertically...

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    Understand, your vertical idea might work...but I think you're overstating just how much "downforce" these things provide, versus just reducing underbody lift. I'd be shocked if there's more than 50-75 pounds vertical force total on that plane...time for a fish scale, string, and GoPro camera?
    Not my circus...not my monkeys...

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    might be. the good news is that the exposed area in the front builds a lot of positive pressure on top, which could be creating moment about the forward pin, and actually negating a lot of the loading to the rear pin from whatever low pressure there is.

    I think an average pressure drop underneath + front end high pressure area is producing ~150-200# on the front wheels of our little MR2s, just based on the bowing of the 3/4" hardwood plywood planks we use. we don't have good ride height change data. But there's a LOT of exposure under the beak of an MR2, so it might be a more extreme example (though most of you all are seeing 50% higher top speeds and should expect a LOT higher loading for a given shape).
    EDIT:
    throwing some math at it, and rough numbers for all variables, making assumptions etc... the splitter on our MR2, which has ~6in of distance from the tip to the first support bolt would take ~20psi of pressure differential to deflect the tip 0.1in.
    Damn I wish I had me some CFD...
    Last edited by Chip42; 03-07-2014 at 05:57 PM.

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    Standard 1/8" aluminum pop rivits sheer way less than even machne screw limit (memory says 250 - 400 lbs) But they'd also be subject to fatigue too. I'm thinking nylon bolts, but didn't see any at Home Depot to try. When I was a tween we used a single grade 8 1/4-20 as a sheer for our New Holland hay bailer. Once my dad tried a grade 5 (all he could find) but it was too ductile and sheered on start up. Lesson learned, strong but brittle failure was needed.

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    I used to vacuum form parts for a design studio I worked in. A splitter is going to be tough in that you'lll need to heat a large sheet, which means a large oven or other heat source, and secondly, you'll need a pretty good vacuum and a nice big and flat frame to house the material. It's all probably cake with the right tools, but for us homebuilders, it's a tricky operation.
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    Chip, I've decided to take your advice and mount brackets from the aluminum bumper bar to the splitter. The front is a slotted pin and the rear is a nylon bolt for a sheer element. The final peice will be slotting/clearancing my new finder liners where they normally mount to the bumper cover and under tray.



    The pin is formed from a 1/4 bolt and a 3/8" pin with 1/4 washer on the outside, the rear hole is for a 1/4" nylon bolt while the wood block is to prevent the angle from stabbing a tire (I'm going to trim the excess bolt off.



    The bracket from the bumper bar to engauge with the pin in the rear facing slot and the nylon bolt with the rear hole. I made these out of steel angle because it's easier for me to weld angle than to machine aluminum.



    My splitter, ready to be sealed and painted. I decided to eliminate the center cut back and go with a straight planform as the car's not likely to get sucked down to bad with 8oolb front springs and just over 3" of ground clearance.
    Last edited by Z3_GoCar; 03-29-2014 at 01:03 AM.
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    So, I used my new splitter at Thuderhill, worked well on Friday, Saturday, and half of the race on Sunday. Then in the middle of the race on Sunday, I went off at the back side of the hill at turn 5. The right side sheared the 1/4"x20 bolt but the left side held and bent the angle bracket, but tore the plywood off the carrage head bolts, but there's no damage to the nose of the car. So I guess it worked as planned.
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    :thumbsup: I think?
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    What was the material of the bolt that didn't shear?
    Marty Doane
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